The present invention generally pertains to injection molding and is particularly directed to improved insulated runner injection molding methods.
An insulated runner system includes mold-cavity-encasing mold parts and channel-encasing mold parts. The mold-cavity-encasing mold parts encase the mold cavities; and the channel-encasing mold parts encase a channel, in which injected plastic material forms an insulated runner with a solidified insulating plastic shell and a molten plastic core for conducting injected molten plastic material toward the mold cavities. A preferred method of operating and controlling an insulated runner injection molding system includes the step of:
(a) injecting molten plastic material into the channel to form an insulated runner in the channel and into the at least one mold cavity in accordance with parameters of a molding sequence including a plurality of injection molding cycles. PA1 (a) injecting a predetermined quantity of molten plastic material into the channel to form an insulated runner in the channel; PA1 (b) injecting further molten plastic material through the insulated runner into the at least one mold cavity in accordance with parameters of a molding sequence including a plurality of injection molding cycles; PA1 (c) subsequent to removal of the sprue, closing the machine gate; PA1 (d) in automatic response to a manual operation incident to said closure of the machine gate, adjusting the molding system in order to enable further said steps (a) and (b); PA1 (e) sensing said adjustment of the molding system; and PA1 (f) in automatic response to sensing said adjustment of the molding system, metering said predetermined quantity of molten plastic material for injection during a further said step (a). PA1 (a) injecting a predetermined quantity of molten plastic material into the channel to form an insulated runner in the channel; PA1 (b) injecting further molten plastic material through the insulated runner into the at least one mold cavity in accordance with parameters of a molding sequence including a plurality of injection molding cycles; PA1 (c) subsequent to removal of the sprue, re-engaging the channel-encasing mold parts; PA1 (d) sensing said re-engagement of the channel-encasing mold parts; PA1 (e) in automatic response to sensing said re-engagement of the channel-encasing mold parts, adjusting the molding system in order to enable further said steps (a) and (b); PA1 (f) sensing said adjustment of the molding system; and PA1 (g) in automatic response to sensing said adjustment of the molding system, metering said predetermined quantity of molten plastic material for injection during a further said step (a).
The core eventually solidifies to form a sprue when the molding sequence is interrupted or after a run of normal production cycles. Upon occurrence of full solidification of the insulated runner to form the sprue molten plastic can no longer be injected through the insulated runner channel, whereby the injection molding system automatically ceases operation, an alarm is provided to alert a system operator to such occurrence, and an end-of-sequence signal is provided. The system operator responds to the alarm and/or a status indication provided pursuant to the end-of-sequence signal by performing a series of steps required for removing the sprue from the channel-encasing mold parts and enabling the molding system for another molding sequence including a plurality of injection molding cycles.
Initially, the operator causes the mold-cavity-encasing mold parts to be disengaged. Then the operator causes the opening of a machine gate that controls access to a space between the disengaged mold-cavity-encasing mold parts, whereupon the operator inspects the disengaged mold-cavity-encasing mold parts and causes any debris to be removed from the disengaged mold-cavity-encasing mold parts.
Subsequent to inspection of the disengaged mold-cavity-encasing mold parts, the operator causes the closing of the machine gate, and then causes re-engagement of the mold-cavity-encasing mold parts. Then, after causing the machine gate to be opened, the operator causes the molding system to be adjusted to enable disengagement of the channel-encasing mold parts. Then, after causing the machine gate to be closed, the operator causes the channel-encasing mold parts to be disengaged. The operator then causes the opening of the machine gate, whereupon the operator causes the sprue to be removed from the channel-encasing mold parts. The operator then inspects the disengaged channel-encasing mold parts and causes any debris to be removed from the disengaged channel-encasing mold parts.
Subsequent to removal of the sprue and inspection of the disengaged channel-encasing mold parts, the operator causes the closing of the machine gate, and then causes re-engagement of the channel-encasing mold parts. Then, after causing the machine gate to be opened, the operator causes the molding system to be adjusted to prevent disengagement of the channel-encasing mold parts. Then, after causing the machine gate to be closed, the operator causes plastic material to be injected into the insulated runner channel and into the mold cavities in accordance with the molding sequence having the predetermined plurality of injection molding cycles.
The operator causes the above-described steps other than the inspection steps to be performed by selectively operating a myriad of control buttons that actuate various mechanisms.
For many years hot runner injection molding methods and systems have been favored over insulated runner injection molding methods and systems for most injection molding applications. In a hot runner injection molding system, the mold parts that encase a runner-system channel, in which injected molten plastic material flows from an injection unit to product-forming mold cavities, are heated in order to maintain the plastic material within the runner-system channel in a molten state.
There is an inherent inefficiency in hot runner injection molding. A substantial amount of electrical energy is required to heat the runner-system channel, and heat leaks from the channel-encasing mold parts to the mold parts that encase the mold cavities to thereby heat the mold cavities and retard the cooling required to solidify products formed in the mold cavities. Consequently, another substantial amount of electrical energy is required to cool a coolant that is circulated in the mold-cavity-encasing mold parts to counteract the heat that is leaked from the heated channel-encasing mold parts. Even with such counteractive cooling, the heat leaked from the channel-encasing mold parts still retards cooling of the products formed in the mold cavities to such an extent as to substantially increase the duration of each molding cycle.
There has been a long felt need to overcome the above-described energy-loss and cooling inefficiency problems incident to hot runner injection molding. Even though these inefficiency problems can be overcome by utilizing an insulated runner injection molding system, since operation of an insulated runner injection molding system does not require that the channel-encasing mold parts be heated, the state of the art of insulated runner injection molding has had various perceived problems associated therewith, as will be discussed below, such that insulated runner injection molding is not currently in common use and is largely thought of by those of ordinary skill in the art as a thing of the past.
The literature available to persons wishing to learn about the art of insulated runner injection molding is contradictory and sometimes misleading, such as in the following examples.
Temesvary "Mold Design for High Speed Production of Disposables", SPE Journal, February 1968--Vol. 24, page 25, states at page 27, "The advantage of the insulated runner system lies mainly in its simplicity and strength. Its disadvantage is that with every shut down, the solidified runner must be removed and, of course, startup is more critical and difficult than start up of the hot runner mold." However, Filbert, Jr. and Williams, "Runnerless Mold Design", Technical Report 196, E. I du Pont De Nemours & Co., Inc., Wilmington, Del., 1977, under the heading, "Insulated Runner Molds" at page 3 state, "(S)hould the internal runner freeze solid, the runner can be removed quickly (at the parting line), and molding resumed with little lost time."; and Dym, "Injection Molds and Molding", Second Edition, Van Norstrand Reinhold, New York, 1987, states at page 230, "Quick-acting latches and movement of the press are employed to accomplish the removal of (insulated) runners with little delay." (parenthetical text added).
Pye, "Injection Mold Design", Fourth Edition, Longman Scientific & Technical, Harlow, 1989, states at page 502, "This technique (insulated runner molding) is only practicable because thermoplastics have good insulating properties." (parenthetical text added). However, Dym, supra, states at page 230, "It (resin for an insulated runner) should have a low specific heat and a high thermal conductivity so that it can be melted quickly and attain temperature uniformity." (parenthetical text added).
Csaszar, "Runnerless Molding Without Hangups", SPE Journal, February 1972--Vol. 28, page 20, states at page 21, "Except for the relatively large runner diameter, the insulated runner in no way differs from runners in other systems." However, U.S. Pat. No. 5,069,615 to Schad et al. states at column 8, lines 4-12, "By having the (insulated) runner systems cut in the face of the plates (114, 116), the ability to machine smoothly curved runner passages is greatly facilitated. As a result, sharp comers and other undesired runner features founded in hot runner channels which cause the resin to hang up and degrade are eliminated." (parenthetical text added).
Dym, supra, states at page 230, "The insulated runner, although limited to certain materials in application, involves lower mold costs and a minimum need for temperature controls." However, Csaszar, supra, states at page 22, "The probe is heated by a cartridge (point 13). In a runnerless (insulated runner) system where more than one probe is used, each probe has its own heating element. Each heater in turn is individually wired its own variac control, insuring the fine tuning necessary to balance between freeze-off and drooling at the gate. Individual control of heat to each probe is absolutely essential to proper function of a runnerless system." (parenthetical text added).
Dym, supra, states at page 229, "The insulated (runner) manifold also consists of a manifold that is fed by a machine nozzle except that the passages are not heated." (parenthetical text added). However, Filbert, Jr. and Williams, state at page 7, "(A)dditional heat should be provided to the (insulated) runner plates." (parenthetical text added).
U.S. Pat. No. 3,520,026 to Stidham et al. states at column 1, lines 31-32, "The insulated runner mold uses a large diameter runner with no heaters of any type." However, "Plastic Mold Engineering Handbook", Fourth Edition, Edited by DuBois and Pribble, Van Norstrand Reinhold, New York, 1987, states at page 374, "An insulated runner mold is a mold utilizing electrical heating elements in hot tips at the cavity gate points in conjunction with a colder manifold section."
Menges and Mohren, "How to Make Injection Molds", Second Edition, Hanser Publishers, Munich, 1993, state at page 208, "The danger that cold material may be carried along from frozen sections is a disadvantage of insulated-runner molds. If it should happen, it lowers the quality of parts. High quality technical parts should therefore be produced with hot-runner molds"; and Dym, supra, states at page 230, "(T)he hot runner manifold affords greater ability to controlling melt temperature, which is a prerequisite for precision and quality of the parts." However, Filbert, Jr. and Williams, supra, state at page 4, "The (insulated runner) system has been used successfully with the entire Du Pont First Family of Engineering Plastics, which include `ZYTEL`* nylon resins. `DELRIN`* acetal resins, glass-reinforced `ZYTEL`, mineral filled `MINLON`* thermoplastic nylon resin, and `RYNITE` thermoplastic polyester resins." (parenthetical text added); and U.S. Pat. No. 5,069,615 to Schad et al. states at column 2, lines 37-41, "Insulated runners have been used in the past because they provide superior streamlining of the runners so that there is less degradation of material when compared to the plastic material in the channels of a hot runner manifold."
Menges and Mohren, state at page 207, "The insulated runner (FIG. 208) operates satisfactorily with materials that flow easily and have a broad melt-temperature range such as PE, PS."; and Filbert, Jr. and Williams, supra, state at page 3, under the heading, "Insulated Runner Molds", "These simple low cost molds, shown in FIG. 2, are generally used with styrenes, ABS, and lower melting ionomers, EVA, or polyolefins which have a broad range of processing temperatures." However, U.S. Pat. No. 5,069,615 to Schad et al. states at column 8, lines 8-12, "(H)eat sensitive resins like PVC, nylon, and the like, normally difficult to process in hot runners, can be easily used in this type of (insulated) runner."
In the opinion of some "experts" the insulated runner system has many limitations to its use:
Csaszar, supra, states at page 21, "Runner diameter (point 2) is critical, however: if the width of the (insulated) runner is too narrow, the melt is likely to freeze off and solidify; if it is too wide, considerable clamping pressure will be required to prevent flashing." (second parenthetical text added).
Dym, supra, states at page 230, "To be considered suitable for insulated (runner) manifold, the materials must have a broad range of melt temperature, must not degrade under prolonged heat exposure as is the case with the insulating `tube`, must have `long flow` properties, and in general must not discolor under these conditions of operation. The material used in this system must have flexibility in setting conditions and allow delays in cycling without thermal degradation. . . . The material (used in the insulated runner system) should also have a high heat-deflection temperature, so that it may set up (cure) in the relatively warm cavity in a short time for economical cycles." (first and second parenthetical text added).
Menges and Mohren, supra, state at page 207, "It is important that the amount of hot material in the (insulated) runner is smaller than the shot weight. Only then will the material in the runner be renewed shot after shot. . . . Before molding begins, (insulated runner) molds should be heated up to about 150.degree. C. Otherwise a start-up of molds especially after a long interruption is not possible. As soon as the thermal balance of the mold has been reached, the heating has to be turned off to allow solidification and demolding of the parts." (parenthetical text added).
Pye, supra, states at page 503, "The (insulated runner) mould needs to be fairly warm when starting up, but to achieve an economic cycle the temperature is then progressively reduced until the required conditions are reached." (parenthetical text added).
Filbert, Jr. and Williams, supra, state at page 4, "(H)eated probes (for insulated runner systems) prevent gate freeze-off and are necessary for crystalline resins that have rapid freezing characteristics." (parenthetical text added).
U.S. Pat. No. 3,021,568 to Scott states at column 2, lines 42-46, "The invention (insulted runner molding) is particularly applicable to materials which can be defined as high density, highly crystalline solid polymers, although low density, low crystallinity polymers can also be employed." (parenthetical text added).
U.S. Pat. No. 3,740,179 to Schmidt states at column 1, lines 37-42, "In these (insulated runner) system, particular care must also be taken to ensure that a relatively thick layer solidifies on the inner wall of the insulated runner during the operation, as a result of which a thicker cross section than normal has to be provided to allow for the passage of the melt." (parenthetical text added).
In the opinion of some "experts" the insulated runner system has many disadvantages to its use:
U.S. Pat. No. 3,520,026 to Stidham et al. states at column 1, lines 36-40, states, "Such (insulated) runner systems . . . suffer from certain disadvantages. For instance, it is difficult to control the temperature accurately and therefore the operation frequently results in the production of drool at the gate or else freezing off at the gate." (parenthetical text added).
U.S. Pat. No. 3,740,179 to Schmidt states at column 1, lines 31-37, "(D)isadvantages are associated with the use of an insulated runner. More particularly a drop in temperature and pressure occurs in the vicinity of the insulated runner. In addition to this, the volume of the charge for the relevant injection-moulding machine must be large enough for the gage system and the mould cavities to be filled with the initial charge."
U.S. Pat. No. 4,072,737 to Wolf states at column 1, lines 22-31, "One particularly attractive system involves the use of an insulated runner, however, the use of a cartridge heater or heated torpedo extending into the gate channel as commonly used in such a system, creates significant lean and distortion problems when reheating molded preforms. This lean causes thermo-formed articles prepared from such preforms to have an undesirable variation in wall thickness and often the minimum acceptable limits will not be satisfied."
U.S. Pat. No. 4,965,028 to Maus et al. states at column 4, line 40 to column 5, lines 11, "Such (insulated runner) systems obviously have as an inherent disadvantage a gross thermal inhomogeneity within this melt delivery system; . . . By definition, the solidified portion is well below the melting-point temperature of that particular polymer, and the central passageway temperature must be sufficiently above that melt-point temperature to provide easy flow with minimal restriction. Therefore, a temperature gradient between these two extremes of 100.degree. F. is quite common and, consequently, as each cycle shot is delivered, sweeping through all variety of partially-solidifying, high-viscosity materials, along with very low-viscosity, high-temperature material, a very inhomogeneous mix of relatively-poor melt quality is the result." (parenthetical text added).
The present thinking in the art regarding utilization of insulated runner systems is expressed by the following statements:
U.S. Pat. No. 5,554,395 to Hume et al, at column 1, lines 47-58, states, "It, therefore, will be understood that the insulated (runner) apparatus is limited to manufacture of thin walled articles (i.e., articles having a small, comparatively fast solidifying volume) in a fast cycling mold. In addition, the length of the various runner channels must be comparatively short in order to avoid `freeze off` during the low melt pressure interval. Accordingly, such apparatus are limited to use with molds containing a limited number of cavities. In view of these drawbacks, the insulated apparatus is not currently in common use by production molders." (first parenthetical text added).
U.S. Pat. No. 5,551,863 to Hepler, filed in 1994, states at column 2, lines 31-51, "Very few of these (insulated runner molds) are built today because other runnerless molding technologies perform much better than this type. . . . While easy to build, this style of mold was extremely difficult to run, particularly when cycle interruptions occurred. If new material was not frequently introduced into the system, the insulated runner would freeze, and the cull (sprue) would have to be physically removed from the mold. As this was a frequent occurrence, the runner plates were latched together, and the machine clamping pressure was relied on to keep the plates from separating under injection pressure. While successful under some circumstances, like fast cycles and large shots with particular plastics, this type of mold largely is a thing of the past." (parenthetical text added).
Accordingly, the level of ordinary skill in the art of insulated runner injection molding has now declined to an almost unrecognizable low level.